Silicone rubber

Definition (what it is)

Silicone rubber is a family of elastomeric polymers whose backbone is composed of siloxane (–Si–O–Si–) linkages with organic side groups (commonly methyl, phenyl, vinyl, or fluorinated groups). It is an inorganic–organic polymer system supplied as solids, liquids, foams/sponge, or gels that are crosslinked (vulcanized) to form elastic, chemically stable materials. Commercial forms include liquid silicone rubber (LSR), high-consistency rubber (HCR or solid silicone), and room-temperature-vulcanizing (RTV) systems.

Key technical characteristics

  • Wide service temperature range: typically −50/−60 to +200–230 °C depending on grade; some withstand short-term peaks to ~250–300 °C; excellent low-temperature flexibility due to very low glass-transition temperature.
  • Environmental durability: exceptional resistance to UV, ozone, weathering, and moisture with minimal aging and embrittlement.
  • Chemical resistance: inert to water and many chemicals; good resistance to many oils and coolants. Standard PDMS-based grades swell in fuels and many nonpolar solvents; fluorosilicone (FVMQ) grades improve resistance to fuels and solvents.
  • Electrical properties: high dielectric strength, low and stable dielectric constant and dissipation factor across temperature and frequency; suitable for insulation, potting, and high-voltage sealing. Track-resistant grades are available for outdoor insulators and HV components.
  • Mechanical behavior: high elasticity and resilience with generally moderate tensile and tear strength compared to some organic elastomers; excellent compression set resistance; hardness spans from very soft gels (Shore 00) through Shore A ~10–80.
  • Flame behavior: many formulations are self-extinguishing with low smoke/toxicity; flame-retardant grades can meet standards such as UL 94 V-0.
  • Permeability: relatively high gas and vapor permeability versus many organic rubbers (useful for venting; limiting for barrier applications).
  • Biocompatibility and cleanliness: intrinsically inert with low extractables; available in medical-, food-, and low-volatile/low-ionics grades.
  • Adhesion and sealing: highly conformable; can be formulated for primerless adhesion to metals, plastics, and glass; also supplied as sealants and formed-in-place gaskets.
  • Optical and surface options: available in optically clear LSR grades for lenses and light guides; surface feel and friction can be tuned with additives or coatings.

Types and cure systems

  • Base polymers and modifiers: typically polydimethylsiloxane (PDMS); phenyl groups improve low-temperature and radiation performance; vinyl functionality enables crosslinking; trifluoropropyl groups yield fluorosilicones (FVMQ) for fuel/solvent resistance.
  • Reinforcement and fillers: fumed or precipitated silica for strength; alumina, boron nitride, or aluminum nitride for thermal conductivity; carbon or silver for electrical conductivity/EMI shielding; flame retardants (e.g., ATH/MDH); pigments and processing aids.
  • Cure chemistries:
    • Addition-cure (platinum-catalyzed hydrosilylation): two-part systems common for LSR and many RTV-2; fast cure, low shrinkage, no small-molecule byproducts; sensitive to catalyst poisons (e.g., sulfur, amines, tin, some nitriles).
    • Peroxide-cure (radical): widely used in HCR/HTV; robust and cost-effective; often requires post-cure to remove volatiles and optimize mechanical/electrical properties.
    • Condensation-cure: one- or two-part RTV systems that release small-molecule byproducts (e.g., acetic acid, alcohols, oximes); widely used as sealants, coatings, and potting materials.

Manufacturing and processing

  • Liquid injection molding (LIM) of LSR for high-precision, high-throughput parts such as connector seals, keypads, microgaskets, and optical components.
  • Compression, transfer, or injection molding of HCR for O-rings, diaphragms, boots, and vibration isolators.
  • Extrusion of profiles, tubes, hoses, and cable insulation, often followed by continuous vulcanization and optional post-cure.
  • Calendering/sheet production for die-cut gaskets and pads.
  • Foaming/expansion to produce open- or closed-cell silicone sponge/foam for lightweight, low-clamp-force seals.
  • Dispensing and coating for formed-in-place/cured-in-place gaskets (FIPG/CIPG), potting/encapsulation, and conformal coatings.
  • Surface preparation and bonding commonly use plasma, corona, or primers to enhance adhesion where needed.

Applications (selected)

  • Seals and gaskets: static/dynamic seals, O-rings, profile gaskets, enclosure and glazing seals in automotive, aerospace, industrial, and construction.
  • Electrical and electronics: wire and cable insulation, HV insulators and connector boots, potting/encapsulation gels, conformal coatings, keypads and switch membranes, LED optics.
  • Thermal management: thermally conductive silicone pads, gap fillers, and interface materials (typically 1–8 W/m·K with ceramic fillers) for power electronics, batteries, and LEDs.
  • Transportation (including EVs): battery pack gaskets and vents, busbar and BMS encapsulation, high-voltage connector seals, inverter/charger TIMs, NVH dampers and mounts, hoses and diaphragms exposed to temperature extremes.
  • Medical and food-contact: tubing, catheters, valves, seals, and molded components where biocompatibility and sterilization resistance are important.
  • General industry and consumer: weatherproofing sealants, appliance and lighting gaskets, cookware/bakeware, and EMI shielding gaskets (with conductive fillers).

Limitations and considerations

  • Mechanical strength and wear: lower tear/tensile strength and abrasion resistance than some alternatives (e.g., HNBR, FKM); notch-sensitive in thin sections.
  • Solvent/fuel exposure: standard grades swell in many nonpolar solvents, fuels, and some oils; select fluorosilicone for these environments.
  • Gas permeability: high permeability can compromise barrier sealing; may require barrier layers or alternative materials.
  • Adhesion: low surface energy can limit adhesion; primers or surface treatments are often needed. Migration of low-molecular-weight siloxanes can contaminate surfaces and affect paint/coating adhesion.
  • Outgassing/volatiles: manage with low-volatile grades and post-cure for sensitive electronics/optics.
  • Cure sensitivities: addition-cure systems can be inhibited by certain contaminants; peroxide-cured parts may need post-cure.
  • Cost and thermal conductivity: generally higher cost and inherently low thermal conductivity unless filled.

Synonyms and related terms

  • Silicone elastomer; polysiloxane elastomer; silicone rubber (SiR); VMQ (methyl vinyl silicone rubber); FVMQ (fluorosilicone rubber); LSR (liquid silicone rubber); HCR or HTV (high-consistency/high-temperature-vulcanizing silicone); RTV (room-temperature-vulcanizing silicone); silicone foam/sponge; silicone gel; silicone potting compound; thermally or electrically conductive silicone.

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